1. Field Of The Invention
This invention relates to subscriber loop telephone service and, more particularly, to range extenders requiring no adjustments for use with diverse subscriber loops.
2. Description Of The Prior Art
Range extenders for telephone loops are known by means of which the conventional telephone service is provided to subscribers over loops having lengths greatly in excess of normal telephone loops. This is accomplished by providing voice frequency gain, sensitive detectors for off-hook and dialing currents, means for repeating the supervisory signals to the central office at increased current levels, and means for matching the impedance of long and sometimes loaded subscriber loops. Another use of range extenders is the substitution of higher gauge wire on subscriber loops, reducing the cost of the copper and compensating for the higher resistance with the range extender. Prior art range extenders are shown in J. L. Henry et al U.S. Pat. No. 3,508,009, granted Apr. 2, 1970, J. L. Henry et al U.S. Pat. No. 3,671,676, granted June 20, 1972, J. M. Nemchik U.S. Pat. No. 3,784,756, granted Jan. 8, 1974, and in the copending application of H. W. Ott Ser. No. 602,988, filed Aug. 8, 1975, now U.S. Pat. No. 4,004,102.
These prior art range extenders have heretofore required manual adjustment of gain and/or impedance matching characteristics in order to render them suitable for use with subscriber loops of varying lengths and wire gauge. This requirement for manual adjustment increases the cost of installation and initial setup of such range extenders when moved from loop to loop and, moreover, increases the risk of improper adjustment on a particular loop. The REG described in the above-mentioned copending application of H. W. Ott, although it requires no manual adjustment of the impedance matching network, nevertheless includes a complicated and expensive impedance detecting circuit to control complex impedance matching networks. This complex circuitry increases the size and cost of the range extender, thereby reducing the number of loops over which this device can be used economically.
Since range extenders are sometimes used on subscriber loops of intermediate lengths, and since the range extender circuitry must go through many transient states while setting up and breaking down telephone calls, a danger exists of the negative impedance amplifier being in a high gain state during a transient condition when the central office termination presents an open circuit. Under these conditions the amplifier can become unstable and create oscillations or other undesirable transients which might interfere with normal telephone use.